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Materials Science and Engineering B 176 (2011) 271–275

Contents lists available at ScienceDirect

Materials Science and Engineering B

journa l homepage: www.e lsev ier .com/ locate /mseb

hort communication

reparation of CdS/PU nanocomposite films by simulating bio-mineralizationrocess and its sensing properties for Ag(I) ions

han Wanga,b,∗, Demei Yua, Yi Huangb, Jinchan Guob, Yongsheng Weib

Department of Applied Chemistry, Xi’an Jiaotong University, Xi’an, 710049, Shaanxi Province, PR ChinaSchool of Chemistry and Chemical Engineering of Xianyang Normal University, Xianyang, 712000, Shaanxi Province, PR China

r t i c l e i n f o

rticle history:eceived 21 May 2010eceived in revised form9 September 2010

a b s t r a c t

A pollution-free method to synthesize polyurethane (PU) was used. PU/CdS nanocomposite films weresynthesized by simulating bio-mineralization process. The factors that affect the hydrothermal stabilityand fluorescence properties of the films were studied. Further, the sensing properties of the nanocom-

ccepted 21 November 2010

eywords:anocompositesensor

posite films to Ag(I) ions in water were systematically investigated. A scanning electron microscopyobservation showed that the sizes of the CdS particles are around 60 nm, and the particles are evenlydoped within the PU films. The fluorescence emission of nanocomposite films has been found to be verysensitive to the presence of Ag(I) ions, and a small amount of Ag(I) ions makes the emissions increasedramatically. The emission is hardly affected by other common ions in water except chloride and sul-

itatiosensi

ilver ion fate through their precipdeveloped into excellent

. Introduction

Inorganic/organic polymer nanocomposites have attracted thenterest of a number of researchers due to their synergistic andybrid properties derived from several components, and they haveeen successful applications in many areas [1–3]. Among variousolymer-based nanocomposites, hybridized inorganic semicon-uctor nanoparticles and polymers have the focus of intensive

nvestigations owing to their important nonlinear properties, lumi-escent properties, size effects, and other important physical andhemical properties [4–8]. Many methods have been developed forhe fabrication of semiconductor/polymer nanocomposites [9–16].owever, there are major disadvantages in the preparation of

emiconductor polymer nanocomposites using these methods,ncluding the poor distribution of inorganic nanoparticle sizes andhe poor dispersion of inorganic particles in the polymer host.

The main methods used to detect heavy metal traces are spec-rophotometry, atomic absorption spectrometry, polarography,tripping voltammetry, X-ray method, inductively coupled plasma

tomic emission spectroscopy, and inductively coupled plasmaass spectrometry, among others. With the rapid development of

iomedicine, food, and environmental engineering, a quick, eco-omical, and convenient method has also emerged. In this regard,

∗ Corresponding author at: Department of Applied Chemistry, Xi’an Jiaotong Uni-ersity, Xi’an, 710049, Shaanxi Province, PR China. Tel.: +86 29 33720935.

E-mail addresses: shanwang2005@163.com (S. Wang), dmyu@mail.xjtu.edu.cnD. Yu).

921-5107/$ – see front matter. Crown Copyright © 2010 Published by Elsevier B.V. All rioi:10.1016/j.mseb.2010.11.007

n effects on Ag+ ions. The films are predicted to have the potential to beng films for Ag(I) ions in water.

Crown Copyright © 2010 Published by Elsevier B.V. All rights reserved.

the development of the sensor technique has introduced a newmethod for the analysis of heavy metal traces. Researchers havefound that an ion sensor has high selectivity in heavy metal analy-sis, so it can be used in this field. Accordingly, the effects of a seriesof heavy metals on the fluorescence emission of films were exam-ined. The monovalent silver ion is more toxic to fish than copper ormercury, and it is an extremely effective bactericide.

In this paper, a pollution-free method to synthesizepolyurethane (PU) and an in situ preparation of CdS/PU nanocom-posites in an aqueous system by simulating bio-mineralizationprocess were reported. Compared with the traditional chemicalroutes, the present method does not use toxic H2S as sulfurion source, and all processes were carried out under ambientconditions. The formation of inorganic nanoparticles and thepolymerization of monomers were performed simultaneously.More importantly, the produced inorganic nanoparticles werehomogeneously dispersed in a polymer matrix during the reac-tion. The film sensor shows high selectivity for Ag(I) salts overinorganic ones and other competing metal salts. As a chemicalsensor, not only is the selectivity of the film for silver salts goodbut its response to the salts is also fully reversible. The details aredescribed below.

2. Experimental

2.1. Materials

Cadmium chloride (CdCl2·5H2O), thioacetamide (TAA), and N,N-dimethylformamide (DMF) were supplied by Aldrich and were used

ghts reserved.

272 S. Wang et al. / Materials Science and Engineering B 176 (2011) 271–275

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Fig. 1. The scheme sy

s received. DMF was dried over 4A molecular sieves and was usedithout further purification. All other materials purchased com-ercially were of analytical reagent grade.

.2. Synthesis of PUs that contain Cd2+

A 250 mL three-neck boiling flask was equipped with a mechan-cal stirrer and a nitrogen inlet. In the flask, 11.6 g of H2N(CH2)6NH2

as dissolved into 41.3 mL dimethyl carbonate, and CH3OH (0.7 g)nd CdCl2·2.5H2O (0.9980 g) were then added. Under normalemperature and pressure, the suspension was stirred with a

echanical stirrer for 8 h. Finally, the mixture of products wasried at room temperature. The products were dissolved into 3.5 mLHCH2CH2OH in the 250 mL three-neck flask. With a catalyzer oflCl3 and epoxy resins, the mixture was stirred vigorously under aitrogen atmosphere at 160 ◦C for 1.5 h. The PUs were obtained.

.3. Activation of the quartz plates

A clean quartz plate (0.9 cm × 3 cm) was treated in a solutionhich is a concentrated H2SO4 solution containing 5 wt% of K2CrO4

t 100 ◦C for 10 min [17,18], and then rinsed thoroughly with plentyf water, After the treatment, the plate should be covered with amooth thin layer of water and should be free of dust and othermpurities. The oxidizing treatment should be repeated if the plateid not pass the test and finally dried at 100 ◦C in a dust-free ovenor 1 h, then kept in a desiccator.

.4. Preparation of the PU/CdS nanocomposite films

First, 1.0043 g PU was diluted with DMF. And the pH 10 solutionhat contains TAA was added to the DMF solution; then the mixtureas dispersed by ultrasonic vibration for 15 min. The compound

lide on the activation of the quartz plates (1 in. × 3 in.) was used

s in a series of controllable experiments. The solid was dried inacuum at room temperature for 6 h and kept for further charac-erization. The schematic synthesis of the amido-coated functionaldS nanocrystal and CdS/PU nanocomposite hybrids is shown inig. 1.

is of the hybrid film.

2.5. Sensing of properties the nanocomposite films

In the experiment, the film was first adhered to one inner sideof a quartz cell with a volume of ca. 3.5 cm. Then the solvent witha volume of 2.5 cm was added into the cell. Finally, the spectrawere recorded when the fluorescence intensity remained stableafter injecting the Ag+ solution into the cell.

2.6. The reversibility of the nanocomposite films

The reversibility of the film to Ag+ was examined in a stan-dard way, that is, by first exposing the film to the aqueous solutionof the analyte, recording the maximum emission intensity of thefilm, and then adding a proper amount of Ag+ solution to thesolution; finally, the emission intensity of the film was measuredevery 3 min for five times. After the measurement, the film waswashed with pure water several times. The measurement wasrepeated five times with the same concentrations of the ana-lyte.

2.7. Analysis techniques

Ultraviolet/visible (UV/vis) absorption spectra were taken witha Perkin-Elmer Lambda EZ-221 spectrometer with the scan rangeof 300–750 nm using DMF as the solvent, and all UV/vis sampleswere diluted with DMF for analysis. Fourier transform infrared(FTIR) spectra were recorded on a NICOLET-NEXUS670 spectrom-eter. The samples were ground with KBr crystals, and the mixturewas pressed into a flake for IR measurement. Thermal analysisexperiments were performed using a DSC apparatus operated in theconventional DSC mode at a heating rate of 10 ◦C/min to simulta-neously determine the correlation of temperature and weight lossin nitrogen atmosphere. Scanning electron microscopy (SEM) wasobtained using a JSM-6380 microscope at an accelerating voltageof 100 kV. Fluorescence measurements were performed at roomtemperature on Instruments RF-5301PC fluorescence spectrome-ter.

3. Results and discussion

The synthesis of amido-coated CdS nanocrystals involves thereaction between cadmium and sulfur ions in the presence

S. Wang et al. / Materials Science and Engineering B 176 (2011) 271–275 273

oectmrITtmsfitsatottoanthtearca4

3

mCs(tusCbbC

Fig. 2. UV spectra of the hybrid film.

f amido-group-containing ligands as the organic ligands. Thelectron-deficient atoms of cadmium on the surface of the semi-onductor serve as the binding sites to anchor organic ligands ando hinder further growth of crystal grains, which results in the for-

ation of nanosized crystals. In the report of Tang et al. [19], theeaction had to be performed at elevated temperatures over 10 h.n our case, preparation of CdS nanocrystals by cadmium chloride,AA, and PU (as the ligand) was done in only 15 min even at roomemperature. The result of absorbance at 433 nm (Fig. 2) is in agree-

ent with quantum confinement effects due to particle size. Thetrong absorption peak is assigned to the optical transition of therst excitonic state of the CdS nanoparticles. The maximum absorp-ion band of CdS nanocrystals occurs at 433 nm through UV/vispectra (Fig. 2). The absorption between 300 and 600 nm can bettributed to the embedded CdS nanoparticles. The average crys-allite size is smaller than 30 nm [20]. It is known that the UV/visn the set absorption of semiconductor nanoparticles is attributedo band-gap absorption, and this is relative to the bulk due tohe quantum size confinement effect [21,22]. DMF was used as anrganic solvent to enhance the solubility of the CdS nanocrystals,llowing these nanocrystals to behave as quantum dots since CdSanocrystals are unavailable in aqueous solution without the addi-ion of DMF. The weight ratio of H2O to DMF in the solution alsoas an effect on particle size, which can easily be observed throughhe transparency of the solution. When the weight ratio (H2O/DMF)xceeded 1.5, CdS precipitation was obviously observed. The UV/visbsorption spectra of the CdS nanocrystals prepared with weightatios of H2O/DMF are 2.0, as shown in Fig. 2. With the sameondition of reagent concentration (Cd2+/S2− = 1 mol/0.67 mol), thebsorption peaks of the weight ratios of H2O/DMF 2.0 center at33 nm.

.1. Spectral data

From the FTIR spectra of the pure PU(a), PU/CdS(b), and an inter-ediate of PU(c) (Fig. 3), strong absorption peaks at 1690 cm−1 (�O) show that abundant hydroxyl groups are tethered on the

urface of CdS nanocrystals. The absorption peaks at 1253 cm−1

� C O C), 3350 cm−1, and 1540 cm−1 (� N H) indicate the exis-ence of the intermediate of PU(c). However, the carbonyl group ofrethane is shown in the region of 1725 cm−1. In the case of the

pectrum of PU/CdS, where the characteristic peaks of pure PU anddS are still maintained, in pure CdS, î(OH) stretching and bendingands are observed at 3423 and 1637 cm−1, respectively. It maye proven that the structure of PU was affected by the presence ofdS.

Fig. 3. FTIR spectra (a, the pure PU; b, PU/CdS film; c, Intermediate of PU).

3.2. Microstructure and Formation mechanism of CdSnanoparticles in PU chains

Fig. 4 shows the micrographs of the PU/CdS nanocompositefilms. The direct evidence of the formation of a true nanocompos-ite was provided by the SEM and TEM investigation. In Fig. 4a, themicrographs confirmed that the CdS particles were well dispersedin the PU matrix. As can be seen in SEM, the average size of the CdSnanocomposites was about 60 nm. The TEM image of the CS filmis shown in Fig. 4b. Reference to the figure, it is revealed that thereal size of CdS nanocomposites is less than 30 nm. It is also possi-ble that polymer chains may be bridged by connecting to the samenanoparticle, and a multiplicity of such bridged chains and parti-cles could lead to particle clusterings [23]. We can clearly see thatthe nanoparticles are dispersed in the PU matrix on a nanoscale,which indicates the formation of a nanocomposite in some way.

PU plays an important role in CdS nanoparticles growth. WhenCd2+ ions solution was added into PU solution which being thehydrophobic microdomain, Cd2+ ions can be much easier to coordi-nate with hydrophilic headgroups NH2 of the PU macromolecule.The gradual homogeneous release of H2S from TAA plays anotherimportant role in the fabrication of CdS nanoparticles because TAAas H2S source that it can decompose slowly in aqueous solutionand release S2− ions homogeneously. This ensures a homogeneousenvironment during the reaction procedure [24], and also makesthe reaction carry out in a very low rate to obtain the PU macro-molecule with perfect crystal structure and morphology. As thereactant H2S is supplied smoothly, the CdS nucleation and growthare all slow and well controlled. Subsequently, the dissolved ionscan recrystallize on larger crystallites the PU macromolecule.

3.3. Thermal properties

The DSC curve (Fig. 5) shows that the temperature of the PUendothermic peak at about 88.59 ◦C is attributed to the volatiliza-tion of residual water and the organic solvent. The exothermicpeak observed at 256 ◦C is related to the decomposition of thepolymer [25,26]. The DSC curve shows that the temperature ofthe PU/CdS endothermic peak at about 95.55 ◦C is the volatiliza-tion temperature of residual water and the organic solvent. Theexothermic peak observed at 270 ◦C is related to the decompositionof the polymer. Obviously, this result indicates a strong and uni-

form interaction between PU and nanoparticles [27]. This behaviorwas already observed with different PU composite and has beeninterpreted as has been interpreted as reflecting the glass transitionof the appended polyurethane chains (i.e., somewhat less mobile

274 S. Wang et al. / Materials Science and Engineering B 176 (2011) 271–275

e hybrid film (a, SEM; b, TEM).

tmatsf

3

otba4CtrioTfilSpafs

Fig. 4. SEM and TEM of th

han free ones), without any substantial contribution from macro-olecules [28]. Meanwhile, some small exothermic peaks observed

t 95.55–270 ◦C are due to the heat effect of the oxidation combus-ion of organic substances [29]. Obviously, this result indicates atrong and uniform interaction between PU and nanoparticles. Thiseature also agrees with the previous SEM measurements.

.4. Excitation and emission spectra of the film sensor

As typical semiconductors, CdS nanocrystals exhibit interestingptical properties. Fig. 6 shows the fluorescence emission spec-ra and exciton spectra of the PU/CdS nanocomposite film. As cane seen. Fig. 6, the characteristic emissions for CdS nanocrystalsre about 510 nm, and the excitons for CdS nanocrystals are about33 nm. In our preparation, excess S2− ions (the molar amount ofd2+ was used to ensure a complete conversion to CdS). Usually,he energy and bandwidth of CdS photoluminescence bands areelated to the size. An effect of band broadening may be the changen the surface structure of the CdS nanocrystals and the particle sizef CdS when the CdS nanocrystals are embedded in the PU matrix.he position of the flourescence emission of the nanocompositelms indicates that the size of the real flourescing CdS particles is

ess than 30 nm [20], suggesting that the CdS particles observed via

EM may be aggregates of small CdS particles and that the small CdSarticles may be separated by the organics. The emission intensitiesnd emission profiles of the film, the control, and the medium wereound to have hardly changed, indicating that the plate surface wasteady and should not affect the follow-up measurements.

Fig. 5. DSC of pure PU and the hybrid film (a, the pure PU; b, PU/CdS film).

Fig. 6. Excitation and emission spectra of film in aqueous (�ex = 433 nm;�em = 510 nm).

3.5. Sensing properties of the present film to Ag+

Fig. 7 shows the fluorescence emission spectra of the film asa function of the concentration of Ag+(C(Ag+)/mg L−1: 1) 0.00; 2)

0.042468; 3) 0.084935; 4) 0.16987; 5) 0.297272; 6) 0.467142;7) 0.67948). Clearly, the emission of the film increased signifi-cantly with an increase in Ag+ concentration (C(Ag+) was from 0 to0.67948 mg L−1). Fig. 8 shows the dependence of the fluorescenceintensity (�ex = 433 nm; �em = 510 nm) of the CdS/PU film on the

Fig. 7. Fluorescence of film in various concentrations of Ag+ (C(Ag+)/mg L−1: 1) 0.00;2) 0.042468; 3) 0.084935; 4) 0.16987; 5) 0.297272; 6) 0.467142; 7) 0.67948).

S. Wang et al. / Materials Science and En

Fig. 8. Response of film to various concentrations of Ag+.

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Fig. 9. Reversibility of the film sensor for Ag+.

oncentration of Ag+. The correlation equation was as follows:

= 1017.1c + 247.96 (R2 = 0.9796) (1)

Generally speaking, the increasing process can last for as longs 5 h. Considering that some minerals exist in tap water, ions suchs Cu2+, Mg2+, Na+, Al3+, Ac2−, Pb2+, K+, Fe3+, chloride, sulfate, anditrate were employed to test the sensitivity of the fluorescence ofhe films to different ions. The results show that the common ionsf all the added salts, that is, Cu2+, Mg2+, Na+, Al3+, Ac2−, Pb2+, K+,e3+, and NO3−, did not affect fluorescence behavior. Only relevantnions were involved in the inhibition effect. The impact of sulfatend chloride anions may be explained by the precipitation effect ofhese ions on Ag+ ions.

.6. Reversibility of the response of the film to Ag+

The results are shown in Fig. 9. A close look at the figure revealshat the response of the film to the same concentrations of Ag+ isully reversible. Further, the time needed to reach equilibrium in theesponse is less than 3 min, which is a rather fast response. To test

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gineering B 176 (2011) 271–275 275

the reversibility of the film sensor to Ag+, the film was alternativelyexposed to a solution of Ag+ and pure water, and the correspond-ing fluorescence emission was measured. After each measurementof the salt solution, the film was washed with pure water severaltimes. It was found that the emission of the film could be fullyrestored (Fig. 9).

4. Conclusion

A new fluorescent film sensor for Ag+ was developed by sim-ulating bio-mineralization process synthesizing CdS nanoparticlesinto PU. The fluorescence emission of the film was demonstratedto be sensitive to the presence of Ag+ in the aqueous phase. Amongthem, Ag+ is much more efficient and sensitive to the emission ofthe film. This exceptional result was ascribed to the hindrance effectinduced by CdS particles. To the best of our knowledge, this paperis the first to report that the film sensor is more sensitive to Ag+

and that the detection limit is 6.290 × 10−3 mol/L. Considering thesensitivity, reversibility, and fast response of the present film in thedetection of Ag+, it can be expected that the film may find uses inthe monitoring of Ag+ in water, soil, and even air.

Acknowledgments

The authors are grateful for the financial support ofShaanxi Province, PR China’s Scholarship Council through Grant08JK482,2010JK902. Xianyang Normal University’s ScholarshipCouncil through Grant 08XSYK109.

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